Semiconductor devices are used in a vast assortment of equipment including computers and communication systems. Many of these semiconductor devices have multilayered structures that include semiconductive layers of dissimilar doping. Separate ohmic contacts are frequently made to the different layers of the device.
It is often desirable that the ohmic contacts of the device be produced without subsequent heating. Such ohmic contacts are referred to as being "non-alloyed". Methods for producing non-alloyed ohmic contacts to doped semiconductor surface layers are well known in the art. See, U.S. Pat. No. 5,013,685 to Chiu et al. which is incorporated herein by reference. They are used in semiconductor devices which are susceptible to damage caused by heat.
Photonic semiconductor devices such as vertical-cavity surface emitting lasers and modulators are particularly susceptible to heating during subsequent semiconductor processing. These devices employ highly reflective mirrors made from gold films or silver capped gold films which form laser cavities or produce reflective modulation. These gold and silver films are most effective when applied directly to the surfaces of pristine semiconductor layers (semiconductor surfaces that have not been subjected to heating) without using base layers of chromium or titanium. When used without the base layers, the gold and silver films can buckle when heated. Buckling of these films reduces their reflectivity and leads to reliability problems.
This heat susceptibility presents major obstacles in the fabrication of photonic semiconductor devices. It is desirable to apply the mirrors early in the fabrication process so that they can be used as masks in self-aligned processing steps. But ohmic contacts must be formed at the end and typically at least one ohmic contact must be made to a buried doped semiconductor layer.
Unfortunately, there are no known processes for fabricating non-alloyed ohmic contacts to buried doped semiconductor layers of semiconductor devices. Since buried alloyed contacts are typically fabricated at about 420.degree. C., buckling of the gold or silver mirror films frequently occurs when the second ohmic contacts are fabricated.
Accordingly, a need exists for a method of producing a non-alloyed contact to a buried doped layer of a semiconductor device.